The present disclosure relates to a back side illumination image sensor.
Image sensors are semiconductor devices that convert an optical image into an electrical signal, and are mainly classified as a charge coupled device (CCD) or a CMOS image sensor (CIS).
According to a related art CIS, photodiodes are formed on a substrate through an ion implantation scheme. However, as the size of the photodiode has been gradually reduced to increase the number of pixels without enlarging the chip size, an area of a light receiving section is scaled down so that the image quality is degraded.
In addition, since a stack height is not sufficiently decreased corresponding to the area reduction of the light receiving section, the number of photons input into the light receiving section may be decreased due to the diffraction of light called “Airy Disk”.
In order to solve the above problem, there is provided a back side illumination image sensor, which receives light through a wafer back side to minimize step difference at an upper portion of a light receiving section and avoid light interference caused by metal routing.
FIG. 1 is a cross-sectional view showing the procedure for forming a back side illumination image sensor according to the related art.
According to the back side illumination image sensor of the related art, a light receiving device and metal lines are formed on a front side of a substrate and then the substrate is subject to a back grinding process such that a back side of the substrate can be removed by a predetermined thickness. That is, the back side of the substrate is ground by a predetermined thickness in order to adjust the distance between an external module and an optical lens.
However, according to the back side illumination image sensor of the related art, an SOI (silicon on insulator) wafer is used as the donor wafer having the light receiving device and the circuit section, and then the SOI wafer is bonded to a handle wafer. Then, a back side thinning process is performed with respect to the donor wafer.
The back side thinning process for the donor wafer according to the related art is as follows.
First, a back side grinding process is performed with respect to the donor wafer such that a silicon layer having thickness of several tens of μm may remain on a buried oxide (BOX) layer of the SOI wafer. Then, an etch back process is performed, thereby completing the back side thinning process.
However, according to the related art, the expensive SOI wafer is used as the donor wafer, so the manufacturing cost may be increased.
In addition, according to the related art, as shown in FIG. 1, wafer edge thinning may occur when the back side grinding process is performed with respect to the donor wafer. Thus, when the subsequent etch back process is performed, chip failure may occur at a wafer edge part, so the economical efficiency is significantly lowered.
Further, according to the related art, a wafer center part is exposed to plasma damage when the etch back process is performed with respect to the wafer having thickness of several tens of μm, thereby degrading sensor performance.
In addition, according to the related art, an isolation region is formed only on the back side of the substrate formed with the photodiode, so that cross talk may occur.
Meanwhile, according to another related art, a photodiode may be deposited by using amorphous silicon (Si). Otherwise, after a readout circuitry is formed on a Si-substrate and the photodiode is formed on another wafer, the photodiode is formed over the readout circuitry through a wafer-to-wafer bonding scheme to form an image sensor (hereinafter, referred to as “3D image sensor”). In this case, the photodiode is connected with the readout circuitry through a metal line.
However, according to the related art for manufacturing the 3D image sensor, the wafer-to-wafer bonding must be performed with respect to the wafer having the readout circuitry and the wafer having the photodiode. Due to the wafer-to-wafer bonding, electric connection between the readout circuitry and the photodiode may not be ensured. For instance, according to the related art, the metal line is formed on the readout circuitry and then the wafer-to-wafer bonding is performed to allow the metal line to make contact with the photodiode. At this time, the metal line may not firmly come into contact with the photodiode and there is difficulty in formation of an ohmic contact between the metal line and the photodiode. In addition, according to the related art, a short may occur in the metal line electrically connected to the photodiode. Various studies and research have been conducted to prevent such a short, but complicated processes are required.